The present invention relates to a high density data storage system with tapered optical fiber as optical read/write head, and more particularly, to the optics and structure of an optical head apparatus for high data storage density recording.
Optical disks have recording densities higher by about one order of magnitude than magnetic disks and other like memory devices. Optical disks are expected to play a major role from now on as a large capacity storage memory addressing multimedia applications.
In optical recording, a light beam is used as a multipurpose tool for both marking to and reading information from a recording media. In optical recording, an optical stylus provides a tightly focused spot of light to the recording media. The light is used to read or form marks on the surface of the recording medium. Optical read/write head designs are well described in the book entitled, "The Physical Principles of Magneto-optical Recording" authored by Masud Mansuripur, Cambridge University Press, 1995.
In the field of laser technology, improvements have been made to use laser sources of progressively shorter wavelengths. The trend toward shorter wavelengths is to make recording densities higher than ever before. Aside from the attempts to enhance the recording density of optical disks using laser sources of shorter wavelengths, some engineers are trying to increase the numerical aperture (NA) of objective lenses. The spot size d of a light beam emitted by a laser source on an optical disk surface is given by dividing the wavelength of the beam, .lambda., by the numerical aperture of the objective lens in use, EQU d.about..lambda./NA.
This is because the recording density is inversely proportional to the squared reciprocal number of the spot size involved. According to the above formula, we also see the reason why shorter wavelengths are used to reduce the spot size.
As understood, anyhow it is difficult and not economic to make the lens with its NA&gt;1, so the diffraction limit, which is the smallest spot size of light, is almost the .lambda.. Therefore, in the so-called far field arrangement, NA.about.1, and d.about..lambda..about.1 .mu.m.
Therefore, from one viewpoint, the larger numerical aperture (NA) is desired to obtain the smaller the spot size, while from another viewpoint, even though generally which is smaller than 1, of the objective lens entails some disadvantages. According to classical diffraction theory, EQU .delta..about..lambda./(NA).sup.2
wherein the depth of focus of a lens, .delta., is the distance away from the focal plane over which tight focus can be maintained. As the disk spins under the optical head at the rate of several thousand rpm, the objective lens must stay within a distance of f(focal length) .+-..lambda./2 from the active layer if proper focus is to be maintained. Given the conditions under which drives usually operate, because such lens needs to be stay within a range of .delta. and such .delta. becomes very small if NA gets larger, it is impossible to make the mechanical systems rigid enough to yield the required positioning tolerances.
According to the aforementioned explanation, an intention to use a larger NA for obtaining a smaller d is impractical.
Anyhow, a so-called near field arrangement applied thereto. A proposed method of reducing the focused spot size in an optical disk system involves the use of a solid immersion lens is disclosed in U.S. Pat. No. 5,125,750 by Corle et al.
FIG. 1 is a schematic diagram of an optical recording system employing a solid immersion lens 26, in accordance with the aforementioned '750 patent. In which, an optical system includes a read/write head assembly 17, for reading or writing information on an adjacent recording medium 16, of the type which includes an objective lens 23, for focusing light energy onto a recording medium 16, including a solid immersion lens 26, disposed between the read/write head assembly 17 and the recording medium 16, wherein the solid immersion lens 26, includes a spherical surface facing the read/write head assembly and a flat surface closely spaced from the recording medium 16.
The immersion lens 26, is so placed to reduce the wavelength of light at the surface of the optical disk by a factor of EQU .lambda./n.sub.sil.
Where n.sub.sil is the refractive index of the solid immersion lens. The reduced wavelength allows smaller spots of light to be produced at the surface of the disk (optical recording medium).
The size of the spot of light determines the size of a stored bit of information on the optical disk. If n.sub.sil =2 is used, the spot size of one bit of information is cut in half so that the density of bits is increased by a factor of 4. Thus the simple addition of a solid immersion lens into an optical system can dramatically increase the storage capacity of an optical disk system.
Another advantage of having a smaller spot size is that the optical power density at the surface of the optically sensitive material is higher than in a system without solid immersion lens. A higher power density allows lower power illumination sources to achieve the necessary power densities to activate (or expose) a bit on the disk.
However, this approach has some disadvantages. The first one is the assembly must be mounted on a movable arm so that the optical alignment will keep accurate enough during the movement of the optical head and the disk. This will increase the weight of the movable optical head and so that it becomes very difficult to keep a very narrow gap between the solid immersion lens and the disk. The second disadvantage is the difficulty to mount the solid immersion lens onto the whole optical head because of the air gap between the solid immersion lens and the objective lens.
In order to reduce the weight of an optical head itself, U.S. Pat. No. 5,245,491 (by Horie et al.) discloses an optical fiber type optical head. FIG. 2 is a view of main figure of the '491 patent showing one embodiment of the fiber type of the optical head.
With this invention, an optical fiber type optical head has a stationary portion 120, which condenses a light beam and leads the light beam into an optical fiber 126. One end of the optical fiber is fixed with the stationary portion 120 while the other end of the fiber 126 is fixed to another portion of the optical head, i.e., the movable portion 130.
This approach separates the whole optical system into two portions and makes only one portion onto mounting arm so that the weight of the optical head itself can be reduced.
However, this approach does not solve the problem of spot size so the storage density is still low and the weight is still an issue here because the optics of movable portion 130, including lenses 127 and 129, is still on the movable arm.